1. Field of the Invention
This invention relates to an on-vehicle stepping motor.
2. Description of the Related Art
A stepping motor has hitherto been used for driving an on-vehicle actuator. This stepping motor is adapted to drive a member to be driven based on a signal sent according to the running state of an engine, in such a way as to set the member in a target position. As a method for transmitting a driving force for such a purpose, there has been a widely used method in which an output shaft gear formed on the rotor of the stepping motor is first driven to rotate, and thereafter the thus generated rotation is transmitted to a gear to be driven, which is attached to the member to be driven.
However, if there are no means provided for directly detecting the rotor position of the stepping motor, or the position of the member to be driven by the rotation of the rotor, the operation of initialization must be carried out. Specifically, in order to correct a difference between the position of the member to be driven at the time of a previous engine stop and a reference position at the time of engine starting, for each starting of the engine, the member to be driven is set in the reference position by rotating the stepping motor in one direction and abutting the rotor or the member to be driven to a stopper.
Thus, there has been developed a stepping motor having a mechanism for pressing the member to be driven to the reference position side by an elastic body, or pressing the rotor of the stepping motor to the elastic body, and returning the member to be driven back to the reference position by the pressing force of the elastic body when the electrification of the stepping motor is stopped after the engine stop.
For such a stepping motor, an electrification pattern of the coil of the stepping motor in the reference position is regulated, and the member to be driven must be held in the reference position when the electrification pattern is activated. Consequently, there must be coincidence between the reference position of the member to be driven, and the position of the rotor holding a permanent magnet held in a magnetically stable state at the regulated electrification pattern.
Therefore, the position of a tooth of the gear to be driven, attached to the member to be driven, is inevitably decided at the reference position, and the stepping motor must be assembled such that the position of a tooth of the output shaft gear formed in the output shaft of the stepping motor can agree with the tooth of the gear to be driven in the reference position.
Because of the foregoing constitution of the conventional stepping motor, it is necessary to carry out assembling work and provide devices described below, in order to assemble the stepping motor while the tooth of the gear to be driven, attached to the member to be driven in the reference position, and the tooth of the output shaft gear of the rotor held in the magnetically stable state at the regulated electrification pattern of the stepping motor, are engaged with each other.
First, the member to be driven is fixed in the reference position. Then, by electrifying the stepping motor based on the electrification pattern in the reference position, the permanent magnet is held in a magnetically stable state, and thereby the rotor for holding the permanent magnet is fixed. In this case, the position of the tooth of the output shaft gear when the rotor is held in the magnetically stable state must be verified. In addition, work must be carried out to engage the tooth of the output shaft of the rotor with the tooth of the gear to be driven of the member to be driven.
Thus, devices for holding the member to be driven in the reference position and electrifying the stepping motor must be provided.
The necessities of complex work and various devices have caused a reduction in assembling work efficiency.
The present invention was made to solve the foregoing problems, and it is an object of the invention to provide a stepping motor capable of increasing assembling work efficiency when the tooth of the gear to be driven of the member to be driven, is to be connected to the tooth of the output shaft gear of the rotor of the stepping motor.
In accordance with the present invention, there is provided a stepping motor, comprising: a plurality of coils for switching a direction of magnetism generated by switching electrification; a stator including a plurality of stator iron cores, for forming a magnetic pole by integrating magnetism generated by each of the coils; a rotor adapted to hold a permanent magnet, and rotated by attraction/repulsion between the permanent magnet and a magnetic pole of each of the stator iron cores; a housing provided to integrally cover the stator and the rotor; and an output shaft gear formed in an output shaft of the rotor, and connected to a gear to be driven, of a member to be driven. In this case, the number of teeth for the output shaft gear is set to a predetermined ratio with respect to the number of magnetically stable points per rotation of the rotor, in order to hold the member to be driven in a reference position when the coil is electrified by a regulated electrification pattern.
According to the stepping motor of the present invention, the number of magnetically stable points per rotation by one electrification pattern is obtained by the following equation: (number of magnetically stable points per rotation by one electrification pattern)=(total number of claws of stator iron core)÷(2 poles, N and S, of stator iron core)÷(2 phases, upper and lower, of stator iron core), wherein, the 2 poles, N and S, of the stator iron core are fixed values of 2.
According to the stepping motor of the invention, the number of teeth for the output shaft gear is set equal to the number of magnetically stable points per rotation of the rotor.
According to the stepping motor of the invention, the output shaft of the rotor and the output shaft gear are formed to be integral.
According to the stepping motor of the invention, the output shaft gear is made of a resin material.
According to the stepping motor of the invention, the output shaft gear is made of a metallic material.